In wireless charging systems, multiple power conversion stages process the energy being transmitted from the source to battery (load). System efficiency is defined as the ratio of output power (at the load) to input power (from the source), and is the product of efficiencies of each power conversion stage. Therefore, to maintain high system efficiency, it is beneficial to reduce the number of power conversion stages. It is typically assumed that a DC source provides the input to the system. In reality, the DC source is itself a by-product of multiple conversion stages from the ac mains, further exacerbating the problem.
The power transmitter consists of a chopper circuit that is either a full-bridge or half-bridge configuration, a resonant network composed of some configuration of inductors and capacitors, and a primary coil. The power receiver has a secondary coil, a rectification stage formed of synchronous rectifiers or simple diodes if efficiency is not a concern, and a buck converter (using external filter components) to provide regulation to the battery being charged. Physical separation between the primary and secondary coils changes the characteristics of the resonant network, and therefore power transfer capability.
Many commercial products use the rectifier stage of the receiver in open-loop, followed by a buck converter stage for providing regulation to the load. Switches of the rectifier stage are typically synchronized to the resonant waveform being rectified to improve efficiency over diode rectifiers. A buck converter made up of switches provides regulation to the load. If input power needs to be adjusted, the receiver communicates with the transmitter. However, this approach requires the power receiver to have two stages for regulating the voltage or current of the battery being charged, making it difficult to achieve very high efficiency. Also, the non-isolated DC/DC regulator typically is a buck converter for simplicity which is a hard-switched topology. Therefore, attempts to increase the switching frequency to shrink the inductor and overcome the limitations of magnetic component technology leads to lower efficiency.